What is the interpretation of the qubit labels when drawing a qiskit circuit with 'with_layout=True'?

Question

When using with_layout=True as argument in qc.draw() where qc is a QuantumCircuit object the resulting text includes the mapping from the qubits in the circuit to the physical qubits like this:

ancilla_0 -> 0 ----
q_3 -> 1 ----
q_2 -> 2 ----   ...
q_0 -> 3 ----
meas: 3/====

Which one (q_3 or 1 eg.) is the qubit in the circuit and which one the physical qubit?

Note: I assume the answer can be found at wire_names() of TextDrawing(qiskit/visualization/text.py lines 782 to 789 in version 0.36.2), but I'm not sure if I understand it correctly.

        for reg in self.qubits:
            register = self.bit_locations[reg]["register"]
            index = self.bit_locations[reg]["index"]
            qubit_label = get_bit_label("text", register, index, qubit=True, layout=self.layout)
            qubit_label += ": " if self.layout is None else " "
            qubit_labels.append(qubit_label + initial_qubit_value)```

Answer 1

$a \rightarrow b$ means the virtual qubit $a$ is mapped to the physical qubit $b$. You can easily check that by visualizing the layout of the transpiled circuit using plot_circuit_layout function.

Draw the virtual view:

plot_circuit_layout(transpiled_circ, backend = backend, view = 'virtual')

Draw the physical view:

plot_circuit_layout(transpiled_circ, backend = backend, view = 'physical')

Now, compare both views with the layout information in your circuit plot.